The accounting treatment of the premium received from the supplier would depend on whether the arrangement is considered to be part of the lease or not. If the premium is considered to be part of the lease, it would be within the scope of IFRS 16, Leases. If the premium is not considered to be part of the lease, it would be accounted for under IAS 16, Property, Plant and Equipment.
For the accounting treatment of the rebate, it would depend on whether the amount of the rebate is known or uncertain at the time of the lease agreement. If the amount of the rebate is known and can be reliably estimated at the time of the lease agreement, it should be accounted for as a reduction of the lease liability and the right-of-use asset as part of the initial measurement of the lease. If the amount of the rebate is uncertain at the time of the lease agreement, it should be accounted for as a variable lease payment, and should be recognized as a liability or an asset when the amount becomes known or can be reliably estimated.
In your case, since the rebate may change after the commencement date, it would be uncertain and should be accounted for as a variable lease payment. The best approach would be to recognize the rebate as a liability or an asset when the amount becomes known or can be reliably estimated. It's important to consider that the estimation should be based on the information available at the time of the lease agreement and should be regularly updated as new information becomes available.
It's important to note that it's important to disclose the nature, amount and expected timing of the rebate in the notes to the financial statements.